Fuel assemblies in nuclear reactors cooled by light water, for example pressurized water reactors (PWR) and boiling water reactors (BWR), or fuel assemblies of CANDU reactors, contain elements comprising a zirconium alloy with the property of low neutron absorption in the heart of the nuclear reactor.
In the case of assemblies for PWR-type nuclear reactors, the jacket tubes for the fuel rods and the plates used for production of the spacer grids for the fuel assembly can be made of zirconium alloy, in particular zirconium alloy containing tin, iron, chromium and where applicable nickel, such as alloys Zircaloy 2 or Zircaloy 4. The same applies to the plugs which close the jacket rods at both ends.
Other alloys, such as the alloy known under the commercial name M5, essentially comprising zirconium and niobium are also used for the production of fuel assembly elements in the form of flat or elongated, solid or tubular products.
In general, the zirconium alloys used for the production of fuel assembly elements comprise at least 97% zirconium by weight, the remainder of the composition which represents at most 3% by weight, with the exception of impurities due to the production of the alloy, can comprise various elements and in particular iron, tin or niobium.
Zirconium alloys meeting these conditions in relation to their composition, depending on the temperature and the heat treatment to which they are subjected, can take one or the other of the two allotropic forms of zirconium i.e. the alpha phase, which is the phase of zirconium stable at low temperature with a compact hexagonal structure, or the beta phase, which is the phase stable at high temperature with a cubic structure.
In certain temperature ranges or at the end of certain treatments, zirconium alloys such as the technical alloys used for the production of fuel assembly rods defined above can have a mixed alpha+beta structure.
Tubular products of zirconium alloy are generally produced by extrusion of a rod which is itself obtained from an ingot by forming and where applicable machining operations.
Solid elongated products (bars) are generally produced by hot rolling then cold hammering of the semi-finished products obtained from the ingot.
Normally a large ingot is cast with a diameter for example between 400 and 700 mm, and generally between 600 and 660 mm. The ingot then undergoes the forging operations in a temperature range in which it can be in the α, β or α+β phase (EP-0.085.552 and U.S. Pat. No. 5,674,330). The ingot is β-phase forged at a temperature between 1000° C. and 1100° C., generally around 1050° C. in the case of Zircaloy 4, to obtain an intermediate product such as a bar or a product of square or octagonal section, of which the diameter of the transverse section (or the diameter of the circle circumscribing the transverse section) is between 250 mm and 400 mm. For example, in the case of an octagonal section, this can have a diagonal with length of the order of 350 mm which corresponds to the diameter of the circle circumscribed.
The intermediate product is then α-phase forged at a temperature between 700° C. and 800° C., for example typically at 750° C., until a bar is obtained with a diameter of 100 mm to 250 mm (and typically a diameter of 205 mm).
Then either the bar resulting from the previous forging phase or a block comprising a part of a cut bar, or a rod produced from a block drilled in its axial direction, is hardened from the β phase (typically from a temperature between 1000° C. and 1150° C.).
Finally, to obtain a tubular product a rod is extruded which can either be the hardened rod obtained in the preceding phase or a rod machined from a hardened bar obtained during the preceding phase of the production process.
To obtain an elongated solid product, hot rolling is performed on the hardened bar.
In all cases before the extrusion operation creating the final tubular product or the hot rolling operation creating a small diameter bar, a semi-finished product is produced in the form of a bar, a block or a rod by a production process comprising a first stage of β-phase forging of the starting ingot and a second stage of α-phase forging of the intermediate product obtained at the end of the first β-phase forging stage.
The known transformation process which has just been described comprises a first β-phase forging stage at a high temperature between 1000° C. and 1100° C. After this first forging stage the intermediate product obtained is cooled at least to the temperature for α-phase forging and generally to ambient temperature because the second α-phase forging stage is not performed immediately after the first β-phase forging stage.
The very high temperature forging of the ingot is a costly and delicate process.
Also during heating of the ingot to bring it to a temperature of 1000° C. to 1100° C. before the first forging stage, the intermediate ingot can absorb hydrogen from contact with humid air or water, the hydrogen fixing in the material in the form of hydrides.
In general the presence of hydrides in the material in the form of coarse precipitates is harmful to the cold formability and corrosion resistance of the products.